The invention relates to a method for the pressure treatment of wood and to wood treated by the disclosed method. In the pressure treatment of wood, various active ingredients such as fungicides or other wood preservatives are impregnated deeply into wood through the application of pressure. A well known example of such pressure treated wood is wood intended for outdoor use in fences or decks and impregnated with preservatives to prevent deterioration of the wood through the action of the elements or from insects or microbes.
Although there are various pressure treatment methods for impregnating wood with active ingredients, the Bethel process is one of the better known. In the Bethel process, wood is dried so that its moisture content is substantially reduced. The wood is placed in a vacuum chamber to draw air from the wood. A mixture of water and active ingredients is then injected into the chamber under pressure. Pressures up to 250 pounds per square inch (psi) can be applied. The pressure is removed so that the wood is again subject to atmospheric pressure. The wood then is transferred to a kiln and dried to reduce the moisture content thereby leaving the active ingredient infused and imbedded in the wood.
The prior art of pressure treatment of wood has proven unsatisfactory in several respects. Most pressure treated wood is used outdoors and is exposed frequently to water, which is able to seep into the prior art pressure treated wood. The movement of water in and out of the wood causes two things to occur. First, the water dissolves any water soluble active ingredients and extracts those ingredients from the wood, thereby reducing the beneficial properties the ingredients may have imparted, such as rot prevention or flame retardance. Second, the water causes dimensional instability of the wood, which can take the form of splitting and cracking upon freezing.
An effective active ingredient commonly used for the pressure treatment of wood is Copper Chrome Arsenate (CCA), a heavy metal. The possibility of leaching has caused some persons to criticize the use of CCA.
The problem of leaching of active ingredients from pressure treated wood is recognized in the prior art, and attempts have been made to address the problem. One prior art attempt at a solution is to use polymeric binders to secure particles of an active ingredient to the wood. These polymeric binders typically use aminoplast curing agents that have the undesirable characteristic of generating formaldehyde. Formaldehyde has various undesirable characteristics, such as generating odors. Formaldehyde also is a suspected carcinogen.
It is an object of the present invention to provide a method for pressure treating wood wherein the active ingredients are secured within and to the wood so that the active ingredients cannot leach from the wood and thereby decrease the active life of the pressure treatment. It is a further object of the invention to provide pressure treated wood within which and to which active ingredients are secured so that the active ingredients may not leach from the wood. It is a further object of the invention to improve the dimensional stability of pressure treated wood to reduce maintenance and to increase the service life of the wood. It is a further object of the invention to provide a machined treated wood by-product for use in particle board or oriented strand board to impart the beneficial properties of the pressure treated wood to the particle board and to the strand board.
These objectives are accomplished and the deficiencies of the prior art rectified by infusing or injecting into wood a reactive solid in a water solution using familiar pressure treatment techniques. The reactive solid comprises a chemical compound selected from the class comprising anhydrides and the analogous acids of anhydrides. The reactive solid solution may include active ingredients such as fungicides, mildewcides, bactericides, flame retardants, colorants, and water repellants.
The reactive solid reacts with and forms chemical bonds to and within the cellulosic structure of the wood. The wood treated with the reactive solid has improved dimensional stability and resistance to swelling on contact with water. In tests on southern yellow pine, infusion with anhydrides reduced swelling due to absorption of water to less than three percent.
The objectives of the present invention may be further accomplished by infusing or injecting into the wood a waxy solid having a melting point above the temperature to which the wood will be exposed in use. Suitable waxy solids comprise paraffinic hydrocarbons or a saturated fatty acid. Infusion or injection of the waxy solid is accomplished by heating the waxy solid and the wood to a temperature above the melting point of the waxy solid and then using familiar pressure treatment techniques to infuse the waxy solid into the wood. The waxy solid may be mixed with an active ingredient. The waxy solid solidifies in the wood, forming a barrier to water and preventing leaching of the active ingredients.
The finished wood product differs from other solid-filled wood products such as the wood-polymer composites in that no sensitizing or dangerous ingredients such as acrylated monomers and peroxide or Vaso catalysts typically associated with this type of product are used.
Wood may be pressure treated by infusion either with the reactive solid solution or with the waxy solid. Best results are obtained by using both techniques and by first infusing the wood with the reactive solid solution followed by infusion with the waxy solid.